If you have the VS Seeker, this job becomes extremely easy.
For Attack, Sp. Atk and Speed, you can simultaneously EV train while raking in plenty of cash. On Route 212, Rich Boy Jason and his Luxio offer 2 Attack EVs, Socialite Reina and her Roselia offer 2 Sp. Atk EVs, and rematches with Lady Melissa and her Lopunny offer 2 Speed EVs. All of these battles net around 16,000 Pokedollars each with Amulet Coin, so you can kill two birds with one stone going this route.
If you're not interested in the money, these methods are slightly more efficient for EV training.
For Attack, on Route 214, you can rematch PI Carlos and his 3 Seaking, which offer 6 Attack EVs per battle.
For Sp. Atk, Collector Douglas on Route 214 uses 3 Roselia, which offer 6 Sp. Atk EVs per battle
For Defense, head to Route 221, where you will meet Collector Ivan. Ivan has a party of three Hippopotas, which each yield 1 Defense EV, for a total of 3 per battle.
For HP, Ace Trainer Brandi in B1F of the National Dex Area in Victory Road has a Pachirisu, Azumarill, and Blissey, which offer 1 Speed EV and 6 HP EVs per battle. If you don't want the 1 Speed EV, kill the Pachirisu with a different Pokemon before switching to your training Pokemon. Don't worry, with a Macho Brace, this method only takes 21 battles to get to Max HP, so the Pachirisu nuisance won't be too much of a problem.
For Sp. Defense, head to Route 223, where 60% of surfing encounters result in Tentacruel that give off 2 Special Defense EVs per battle.
Finally, for Speed, on Route 205 North, Fisherman Andrew has a party of six Magikarp that offer 6 Speed EVs total per battle. Combine this with a Macho Brace and you get 12 EVs per battle, and with a Power Anklet you receive 30 Speed EVs per battle.
Source and my own research
Hope this is what you're looking for!
